1. Field of the Invention
The present invention relates to an image forming method and apparatus, or a color image forming method and apparatus.
More specifically, the present invention relates to a color image forming method and apparatus of the electrophotographic type.
Still more specifically, the present invention relates to a color image forming method and apparatus of the electrophotographic type, wherein a plurality of liquid developing devices accommodating liquid developer comprising colored microparticles dispersed in an electrically insulated fluid medium are used to form toner images of different colors, which are electrostatically transferred and overlaid one over another on a transfer medium such as a recording sheet or the like or an intermediate transfer member to produce an overlaid toner image.
2. Description of the Related Art
In image formation via electrophotographic methods, typically an electrostatic latent image is formed on a latent image carrying member such as a photosensitive member or the like via optical image exposure corresponding to a document image or image data, and this electrostatic latent image is developed as a visible toner image, which is subsequently transferred to and fixed on a recording member to produce a final target image.
Developing methods can be broadly divided into dry type developing methods and wet type developing methods; the developers currently most widely used as liquid developers in wet type developing are liquid developers formed of charge controller, dispersion stabilizer, and colored microparticles (toner) comprising mainly pigment and binder resin dispersed in an electrically insulated dispersion medium (carrier fluid). The toner charge is attained by ion absorption of the charge controller, and the charged toner is supplied for developing by the principle of electrophoresis.
The toner used in wet type developing can be finer than toner used in dry type developing because there is no concern of airborne dispersion into the atmosphere, and it is possible to use toner having a mean particle size in the submicron range. The use of such fine toner microparticles is advantageous in producing high resolution images and ease of fixing the toner image.
When forming color images using such liquid developers, toner images of various colors, for example, cyan, yellow, magenta, and black, are respectively formed on an electrostatic latent image carrying member for each color, and the toner image of each color is electrostatically transferred so as to sequentially superimpose said color images one on another on a transfer member such as a recording member or intermediate transfer member to produce a color image via the overlaying of said color toners. When a color image has been formed on an intermediate transfer member, the color image is subsequently thermally transferred onto a recording member to produce the ultimate color image. Furthermore, the toner image of each color may be sequentially formed on a single electrostatic latent image carrying member, then sequentially transferred and overlaid on a transfer member via electrostatic transfer to produce the ultimate color image.
When performing such multilayer transfers, however, the electric field used for transfer becomes difficult to maintain as the toner layers are superimposed on the transfer medium due to the high resistance of the toner, thereby adversely affecting transfer efficiency. The toner charge is particularly high in the case of small size toner particles used in liquid developers, which necessitates the use of a high electric field to achieve electrostatic transfer and results in even greater difficulty in transferring multilayer images due to the even greater difficulty of maintaining the electric field to overlay and adhere sequential toner layers on a transfer medium in multilayer transfers.
An object of the present invention is to provide a color image forming method and apparatus having excellent multilayer transfer characteristics for toner images transferred to a transfer medium in a color image forming method and device of the electrophotographic type using a plurality of liquid developing devices accommodating liquid developers comprising colored particles dispersed in an electrically insulated fluid medium to form toner images of different colors, and sequentially overlay said toner images on a transfer medium via electrostatic transfer to produce a multilayer toner image.